Laser- and Electron-Induced Recrystallization of
Amorphous Zones in Elemental and Compound
Semiconductors
Igor Jenčič*, Eric P. Hollar†, Ian M. Robertson†
*
†
Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
University of Illinois at Urbana-Champaign, Department of Materials Science and Engineering,
1304 W Green, Urbana, IL 61801
Abstract. Spatially isolated amorphous zones in Si, Ge, GaAs, GaP and InP were created by low dose (≈ 1011 cm-2)
50 keV Xe ion implantations. The ion-implanted samples were subsequently irradiated with electrons or photons which
induced recrystallization of amorphous zones. As the electron energy was lowered from 300 keV the crystallization rate
initially decreased and reached a minimum at approximately one-half of the threshold displacement energy. With further
lowering of electron energy the crystallization rate increased, reaching a maximum at 25 keV, the lowest voltage
attempted. Laser-induced crystallization experiments using photons of energy hν = 2.33 eV at 90 and 300 K were
performed on Si, Ge and GaP samples and amorphous zone regrowth was stimulated in all materials. Sub-threshold
electron-induced and laser-induced crystallization shows that displacive mechanisms and point defects are not required
to stimulate regrowth of isolated amorphous zones but that the defects responsible for crystallization are created via
inelastic energy loss processes.
below the atom displacement threshold (145 keV for
Si and 350 keV for Ge8). This demonstrated that low
energy electron beam induced crystallization does not
require the production of point defects in the crystal.
INTRODUCTION
Ion implantation is an important step in the
production of semiconductor components1. However,
energetic ions penetrating the crystal lattice produce
structural damage such as extended defects and
amorphous material. This damage must be removed
before the device can be made operational and the
conventional method of recovery is thermal annealing.
This paper further investigates the sub-threshold
electron-induced crystallization process. This is
accomplished by testing the electron energy effect on
growth of a range of group IV and III-V
semiconductor materials. Photon irradiation is also
employed to learn about the importance of excitation
processes on crystallization.
Recrystallization can also be promoted at relatively
low temperatures by irradiation with a beam of ions2.
The recovery process is in competition with the damage created by the ions. This process is called Ion Beam
Induced Epitaxial Crystallization (IBIEC). Electron
Beam Induced Epitaxial Crystallization (EBIEC)3,4
leaves less residual damage than IBIEC and continues
with no reversal to layer-by-layer amorphization even
when the temperature is lowered to 15 K5. In our
previous work6,7 we have shown that, contrary to
EBIEC of amorphous layers, isolated amorphous
zones disappeared also when the electron energy was
EXPERIMENTAL METHODS
In order to create amorphous material, Si, Ge,
GaAs, GaP and InP specimens were thinned to
electron transparency and then irradiated with
energetic ions. Ion irradiations were done at the
HVEM-Tandem User Facility at Argonne National
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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Laboratory in the Intermediate Voltage Electron
Microscope, a modified Hitachi H-9000NAR9. Xe ions
were chosen because they create large amorphous
zones which simplifies the analysis. The depth and
degree of damage was varied by varying the Xe ion
energy (50 – 300 keV) and dose (5 x 1010 – 1013 cm-2).
Because such a low dose was needed, the lowest
sustainable dose rate (≈ 1010 /cm2sec) was used.
In-situ TEM laser irradiations were performed on a
modified Philips EM420 at the University of Illinois.
A leaded glass window was installed into the former
EDAX port of the EM420, providing a line-of-sight
path to the inserted sample. No electrons touched the
specimen during laser irradiation. The laser used
produced green light (λ = 532 nm) corresponding to a
photon energy of hν = 2.33 eV.
For the electron irradiations, a TEM was used as
both a source of electrons and a tool to record
amorphous zone behavior. Most experiments were
done using a Philips CM30 at Argonne National
Laboratory, allowing electron energies between 25 and
300 keV and electron dosimetry through an in-column
Faraday cup. The evolution of the ion implantation
damage under the influence of electrons was observed
by exposing a specific area of the sample to the
electron beam and recording the image at regular time
intervals. This results in a series of images such as
those shown in Figure 1. The amorphous zones appear
dark on a relatively constant, light background. The
specimens were irradiated and imaged in the bright
field down zone condition10.
RESULTS
Examples of electron induced crystallization, one
each for Si, Ge, GaAs, GaP and InP, are shown in
Figure 1. Each electron irradiation experiment
typically resulted in 8 to 12 micrographs; however, to
avoid overcrowding only 4 images per series are
shown here. Regardless of the material, it is visibly
evident that electron irradiation causes each
amorphous zone to shrink and many to disappear
altogether. All amorphous zones shrink from the edge
in, using the surrounding crystal as seed. No
nucleation and crystallization was observed within the
zone interior.
These images provide visible evidence that subthreshold electron induced crystallization occurs. The
kinetics of the crystallization process were gauged by
zone counting. The number of amorphous zones in a
given area decreases approximately linearly with
increasing electron dose. The slope of the line fitted
through the number of zones vs. electron dose is
denoted the disappearance rate RD and is introduced
as a measure of the crystallization rate.
For all five materials, the energy of the electrons
used to stimulate crystallization was varied from 25 to
300 keV. Figure 2 shows the disappearance rate for
electron-induced crystallization of isolated amorphous
zones in elemental and compound semiconductors
when the energy of the impinging electron beam is
varied. Electron irradiations were done at room
temperature, except in GaAs; where they were done at
90 K to avoid the complication of thermal annealing
which occurs above 200 K11. The disappearance rates
of Si, Ge and GaAs are very similar, especially in the
low energy regime; whereas the rates of GaP and InP
are consistently higher. In all materials the
disappearance rates show the same trend: as the
electron energy is lowered from 300 keV the
disappearance rate initially decreases, reaching a
minimum at an energy, Emin, somewhat below the
threshold displacement voltage, Ed. Lowering the
electron energy further unexpectedly increases the
disappearance rate, the highest rate occurring at
FIGURE 1. Example of electron beam induced
crystallization in Si, Ge, GaAs, GaP and InP. Crystallization
was stimulated by irradiation with 50 keV electrons (for
GaAs and InP, 100 keV) at room temperature except for
GaAs which was at 90 K. The accumulated electron dose
[e-/cm2] is given on each image. Arrows indicate zones
which disappear.
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25 keV, the lowest voltage attempted. From this graph
we can estimate Emin, the energy at which
disappearance rate is a minimum, to be roughly 0.5 Ed.
beam heating was experimentally examined by
electron irradiations of Si and Ge at temperatures from
90 to 300 K. The sample temperature showed no
influence on the disappearance rate. Thermallyinduced growth rates are strongly temperature
dependent and the lack of an increase in the electroninduced disappearance rate when the sample
temperature is increased by 210 degrees confirms that
electron induced crystallization is not a thermal effect.
A possible explanation for the electron-induced
crystallization behavior is that the sample is heated by
the electron beam and consequently the amorphous
zones are being thermally annealed. The calculated
maximum temperature rise is 2.6 degrees7. The role of
Compound Semiconductors
Elemental Semiconductors
D isappearan ce rate [cm 2/e - ]
1.E-21
1.E-21
a)
Si
Ge
1.E-22
b)
1.E-22
InP
GaP
GaAs
E d(G a)
E d(G e)
E d(Si)
E d(As)
1.E-23
1.E-23
0
50
100
150
200
250
300
350
Electron en ergy [keV]
0
50
100
150
200
250
300
350
Electron en ergy [keV]
FIGURE 2. The disappearance rates of elemental and compound semiconductors as a function of electron energy at
room temperature (for GaAs, 90 K). Displacement threshold energies for Si, Ge and GaAs (for both components)
are also shown.
With beam heating eliminated, the next plausible
explanation for low energy electron-induced
crystallization is excitation of electrons in the material.
For low electron energies (< 100 keV), electronic
energy loss scales as 1/E, which is a similar
dependency to that of the recrystallization rate. An
obvious test of the hypothesis that electronic
excitations are critical to sub-threshold electron-
a
induced crystallization is to create these excitations via
other means.
Figure 3 shows the evolution of amorphous zones
in Ge during a in-situ green laser (hν = 2.33 eV)
irradiation. Similar laser irradiations were done on
amorphous zones in Si and GaP. In all materials laser
beam induced regrowth of isolated amorphous zones.
b
c
d
FIGURE 3. A sequence of TEM micrographs of the same area on a Ge sample, exposed to a laser beam (λ = 532 nm,
hν = 2.33 eV) at room temperature. Picture a) was taken before the laser irradiation, b) after 3 hours, c) 7 hours, and d) 15.3
hours of laser irradiation.
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of the low energy electron beam altering material
structure and properties.
DISCUSSION
The experimental results presented in the preceding
chapter demonstrated that electrons with sub-threshold
energies are capable of stimulating crystallization of
amorphous semiconductors (Si, Ge, GaAs, GaP and
InP) and that the same can be achieved also with
photons. Moreover, as Figure 2 shows, the efficiency
of the process increases as the energy of the electron
beam decreases below about 100 keV.
ACKNOWLEDGEMENTS
This work was supported by the Ministry of
Education, Science and Sport (IJ) and US DOE, grant
DEFG02-91ER45439 (EPH and IMR). The use of the
electron microscope facilities at Argonne National Laboratory, and in the Frederick Seitz Materials Research
Laboratory at the University of Illinois is appreciated.
We propose the following model to explain this behavior: A low energy electron (E < Ed ) or a photon
(hν > Eg) is capable of exciting valence shell electrons
into anti-bonding levels, weakening or severing the
bond between two atoms. While in the excited antibonding state, due to the extra strain energy associated
with the c/a interface these dangling bonds will tend to
propagate and promote crystallization in the manner
analogous to that described by Spaepen and Turnbull12. Eventually these defects will be caught in a trap
or destroy each other via recombination and become
localized into a covalent bond. As atoms near the interface shift to their preferred crystalline lattice positions,
the total energy of the system is lowered and the amorphous zone shrinks. The actual atomic rearrangements
are fairly impossible to measure, but MD
simulations13,14 have given insight into these processes.
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